1. The Field of the Invention
The present invention relates generally to cardiac pacing and more particularly to an improved electrode for use in stimulating or sensing electrical activity of the heart and to pacing lead assemblies incorporating such electrodes.
2. The Prior Art
It is well known that the sinoatrial (SA) node of the normal mammalian heart acts as the natural pacemaker by which rhythmic electrical excitation is developed and propagated in the atria. In response the atrial chambers contract pumping blood to the ventricles. The excitation is propagated in the atrioventricular (AV) node, which imposes a delay, and then via the conduction system consisting of the bundle of His and Purkinge fibers to the ventricle muscle causing contraction and pumping of blood from the ventricles. Disruption of the natural pacing/propagation system occurs as a result of aging and disease.
Where normal rate or rhythm is not spontaneously maintaining the heartbeat of a human patient, the condition is corrected typically by utilization of an implantable cardiac pacemaker selected according to the particular deficiency of the patient. In its simplest form, the cardiac pacemaker consists of a pulse generator powered by a self contained battery pack; a lead assembly including an electrode adapted to be positioned in stimulating relationship to excitable myocardial tissue either externally (an epicardial electrode) or internally (an endocardial electrode) of the heart, and an insulated electrically conductive lead interconnecting the pulse generator to the tissue to deliver electrical stimuli to the tissue; and a second electrode connected to a reference potential, by which the electrical circuit is completed via body tissue and fluids. The entire lead assembly is often referred to simply as the lead and the terminology "lead" and "electrode" are sometimes used interchangeably, albeit inaccurately.
The customary lead choice for the implantable cardiac pacemaker is an endocardial lead or leads because it is readily inserted perveniously to introduce the stimulating electrode directly into the chamber of the heart which is to be paced. In contrast, an epicardial lead requires thoracic surgery to affix the electrode to the heart's outer surface. In either case various means are employed to insure maintenance of positioning of the electrode relative to the excitable heart tissue. For epicardial leads, active fixation, such as sutures or a sutureless screw-in electrode, is employed. Endocardial leads may utilize active fixation, such as a corkscrew, or passive fixation, which is less invasive, in the form of flexible barbs or hooks.
In operation, the output pulses from the pulse generator of a cardiac pacemaker are delivered via the lead for electrical stimulation of the excitable myocardial cardiac tissue at or in the immediately vicinity of the site of the cathode to produce the desired rhythmic contraction of the affected chamber. As is well known, stimulation is attributable to current density and hence small area electrodes will suffice inasmuch as the current required to produce a given current density decreases in direct proportion to the active area of the electrode. Small area electrodes (cathodes) therefore serve to prolong battery life resulting in a lengthening of the interval between required pacemaker replacements.
In essence, stimulation requires that an electrical field of sufficient strength and current density be impressed upon the excitable tissue in the vicinity of the cathode site to initiate contraction. The minimum electrical impulse necessary to produce that effect is referred to as the stimulation threshold. The greater the efficiency of the cathode in impressing the electric field on the tissue, the smaller is the amplitude and/or duration of the pulse required to exceed the threshold. Accordingly, high efficiency low threshold electrodes conserve energy and prolong battery life. Some authorities have theorized that because greater electrical efficiency lowers the electrical energy required for stimulation, it is a factor in reducing injury to the tissue at the stimulation site.
The chronic stimulation threshold for a given patient is typically on the order of two to three times greater than the acute threshold observed at the time of implantation and within the first few days thereafter. The increase in threshold is attributed to fibrotic growth; that is the formation of a layer of non-excitable tissue about the electrode tip at the stimulation site. This fibrotic layer creates a virtual electrode surface area which is consistently greater than the actual surface area of the electrode and consequently raises the stimulation threshold. Interestingly, the increase of chronic threshold over acute threshold is proportionately greater (to a limit) as the electrode area is decreased, presumably because the ratio of virtual to actual surface is higher for small area electrodes. Many authorities have speculated that the particular composition of the electrode may contribute to or retard fibrotic growth.
It becomes quite clear from the prior art studies that there are two factors which have a significant impact on the efficiency of the electrode tip. These factors are the shape of the electrode and the composition of matter forming the electrode. The shape of the electrode, as discussed above, should be kept to a minimum size while the surface area should be maximized as the greater amount of contact area allows a direct reduction in the electrical energy usage. The composition of matter making the electrode tip or coating/plating on the tip also will directly effect the efficiency of the transfer of electrical energy to the tissue as well as may have an effect on deterring the fibrotic growth.
There are two patents which are particularly representative of the prior art, namely U.S. Pat. Nos. 4,649,937 to DeHaan et al. and 4,679,572 to Baker. The former patent teaches an electrode tip member which has a rounded or bullet shaped distal end with grooves etched into the end to increase the surface area of the electrode member within the small displaced surface area to minimize polarization of the tip member while insuring sufficient electrical current flow to cause heart muscle depolarization. Preferably the tip electrode member is made of titanium or titanium alloy and is coated with carbon. The Baker patent teaches an electrode for use in cardiac pacemaking having a conductive tip portion including a substrate composed of material conventionally employed for pacing electrodes and a layer or film of iridium oxide overlying the surface of the substrate. The tip portion may be provided with recesses which confine the iridium oxide surface and an iridium oxide layer may be formed on both the cathode and anode portions of a bipolar lead for efficient transduction of the electrode electrolyte interface in the environment of the cardiac pacemaker patient's body.